A 3-way catalytic (TWC) converter controls emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The TWC also stores oxygen. When the listed idle conditions follow the listed driving conditions the Catalyst System Efficiency test will run. The engine control module (ECM) monitors this oxygen storage capacity by comparing the pre and post-catalyst oxygen sensor signals while adding and subtracting fuel during specific idle conditions.

The ECM compares the pre- and post-catalyst oxygen sensor signals to determine if the oxygen storage capacity of the catalyst is degraded.

Important: A new converter with less than 100 miles on it may set P0420 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 hour may correct the condition.

1. Driving Conditions

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DTCs P0030, P0036, P0053, P0054, P0059, P0068, P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0120, P0121, P0122, P0123, P0128, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0151-P0155, P0171, P0172, P0174, P0175, P0201-P0208, P0220, P0222, P0223, P0315, P0300, P0301-P0308, P0335, P0336, P0340, P0341, P0351-P0358, P0401, P0403, P0404, P0405, P0406, P0411, P0412, P0418, P0442, P0443, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0496, P0506, P0507, P060D, P1101, P1133, P1153, P1174, P1175, P1258, P1400, P1404, P1516, P2101, P2119, P2120, P2122, P2123, P2125, P2127, P2128, P2135, P2138, P2176, P2430, P2431, P2432, P2433, P2440, P2444, P2A00, P2A01, or P2A03 are not set.

•	The engine has been running for more than 6 minutes.

•	The vehicle is in Closed Loop.

•	The vehicle has Fuel Trim enabled.

•	The engine coolant temperature (ECT) is between 70-125°C (158-257°F).

•	The barometric pressure (BARO) is more than 70 kPa.

•	The intake air temperature (IAT) is more than -20°C (-4°F).

•	The throttle position (TP) is greater than 2 percent.

•	The calculated TWC temperature (Calc. TWC Temp.) is between 385-640°C (725-1,184°F).

•	The battery voltage is more than 10.7 volts.

2. Idle Conditions

•	The vehicle speed is less than 3.2 km/h (2 mph).

•	The engine speed has been more than 1,200 RPM for 25 seconds.

•	The automatic transmission is in Reverse, Drive, or Low.

•	The TP is less than 2 percent.

•	The short term fuel trim (FT) is between -10 and +10 percent.

•	The engine speed is less than 1,000 RPM.

•	The engine idle is stable.

•	This diagnostic attempts one test during each valid idle period when the above conditions have been met. These diagnostic attempts up to 6 tests during each drive cycle.

DTC P0420 is a Type A DTC.

DTC P0420 is a Type A DTC.

Scan Tool Reference

Electrical Information Reference

DTC Type Reference

Notice: Refer to Heated Oxygen Sensor Resistance Learn Reset Notice in the Preface section.

Notice: Refer to Three-Way Catalytic Converter Damage Notice in the Preface section.

1. If any other DTCs are set, perform those diagnostics first. Refer to Diagnostic Trouble Code (DTC) List - Vehicle .
2. Inspect for the following conditions, which may cause a catalytic converter to degrade:

•	An engine misfire

•	High engine oil or high coolant consumption

•	Retarded spark timing

•	A weak or poor spark

•	A lean fuel mixture

•	A rich fuel mixture

•	A damaged oxygen sensor or wiring harness

•	A catalyst test may abort if the vehicle falls outside the conditions for running the DTC.

•	A catalyst test may be temporarily degraded if a fuel with high sulfur content has been used. Drive the vehicle at highway speeds for 10 minutes and retest the converter.

3. Operate the vehicle within the Conditions for Running the DTC.

⇒	If the DTC fails this key cycle, continue with this procedure.

4. Inspect the catalytic converter for the following conditions:

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Dents

•	A severe discoloration caused by excessive temperatures

•	Road damage

•	An internal rattle caused by damaged catalyst substrate

•	Restrictions

⇒	If a condition is found, replace the catalytic converter.

5. Inspect the exhaust system for the following conditions:

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Leaks

•	Physical damage

•	Loose or missing hardware

•	Properly torqued HO2S

•	Restrictions

⇒	If a condition is found, repair the exhaust system.

6. Inspect the post catalyst HO2S for the following conditions:

•	A grounded wiring harness

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Damage

⇒	If a condition is found, replace the post catalyst HO2S. If no physical condition is detected, replace the catalytic converter.

7. Engine running, observe the DTC information with a scan tool. DTC P0420 should not set.
8. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

1. Clear all DTCs.
2. Allow the engine coolant temperature to reach 70°C (156°F).

•	Place the gear selector in Park, Neutral for manual.

•	Set the parking brake.

3. Press and hold the service brake.
4. Run the engine for 6 minutes at 2,500 RPM.
5. Allow engine speed to return to idle speed, then place gear selector into Drive, or apply clutch pedal for manual.
6. After 2 minutes, verify that DTC P0420 has passed or failed this ignition cycle.

⇒

If not passed or failed this ignition cycle, the test aborted resulting in an inconclusive test result. The above test may need to be repeated up to 5 more times during this drive cycle. The first test should be run after the initial warm up period of 6 minutes. Between subsequent tests, maintain 2,500 RPM for 2 minutes.

7. If 6 tests have been attempted and the DTC has not run or passed during this key cycle, turn the key to OFF for 30 seconds, then run the procedure a second time.